The use of lasers in the fields of communications, medicine and machining is increasing steadily. Continuing progress in these areas imposes ever more stringent requirements on the laser devices themselves and their control mechanisms. Especially high demands are placed on high-repetition laser systems employed in trimming, annealing, surface texturing, cutting, welding, melting, photodepositing, photodissociating, photopolymerization and surgical procedures. In all these applications the processing quality depends greatly on the pulse-to-pulse energy stability and pulse-to-pulse peak power (which is equivalent to "amplitude") stability of the laser system.
Typically, laser pulsing is achieved by various Q-switching techniques. At high repetition rates the temporal separation between the pulses is on the order of, or less than, the time required to restore or pump the lasing medium. This means that the repetition rate approaches or exceeds the inverse of the effective storage time 1/.tau. of the laser medium. In this frequency regime there is less energy stored per pulse and the laser is closer to threshold at the start of each pulse as compared with low repetition rate operation. As a result, the pulse-to-pulse energy stability in this frequency regime is negatively affected.
One approach to solving the problem of initial variability or transience in mean energy pulse stability is presented by Gibbs in U.S. Pat. No. 5,303,248. Gibbs proposes to treat the turn-on transience by negative feedback as used in pulse-width modulation of diodes. This does allow the laser to quickly reach the regime of desired power level per pulse, but does not equalize pulse-to-pulse energy or peak power fluctuations once operation near the desired output level is established.
In the past many techniques have relied on simply attenuating the output pulse train in a time-varying manner to a preset level to remove pulse energy variability. This solution incurs many losses due to the additional elements required and, of course, the energy dumped in the attenuation process itself. Examples of systems implementing a common attenuation technique for achieving pulse-to-pulse energy stability are presented by Wilcox in U.S. Pat. No. 5,157,676.
In U.S. Pat. No. 5,128,601 Orbach discloses a method for stabilizing laser pulses based on an external control or pump control and a compiled table of peak pulse energies. The pulses are effectively attenuated by the control based on the look-up table. This approach is empirical and works only if the laser is pre-calibrated with a statistically significant number of pulses at various time intervals or pulse rates. It is important to realize that this approach does not produce a direct, pulse-to-pulse control for a laser operating at any fixed repetition rate. Moreover, the method can not be applied directly to any given laser and does not involve a direct feedback loop.
Another solution to the problem is advanced by Klaras et al. in U.S. Pat. No. 5,365,532. The inventors disclose how to stabilize the amplitude of a Q-switched laser by cavity dumping. The technique does not gather information about the previous pulse to correct the next. In fact, it is simply concerned with eliminating the intrinsic jitters in the signal. Furthermore, the method is not specifically suited for high-frequency applications.
In U.S. Pat. No. 5,339,323 Hunter et al. teach how to control the mean energy stability from pulse to pulse by adjusting the time between pulses. Such adjustments allow the medium to be properly pumped to a desired stored energy between pulses. In other words, the technique is founded on the realization that a sufficient build-up of flux from one pulse to the next must be allowed to take place. Now, the mean energy can be controlled. Unfortunately, at high repetition rates, e.g., on the order of or larger than the inverse of the effective storage time, i.e., .gtoreq.1/.tau., it is impossible to perform adjustments by temporal shifting of pulses. Therefore, this technique can not be used in the regime of high pulse repetition rates.
The dynamics of Q-switched lasers are well-known to persons skilled in the art. Pulse stability has been improved using injection seeding, intra-cavity etalons and other means. These approaches have benefits, but also drawbacks. Injection seeding is costly and complex. Etalons often induce power losses and can be sensitive to alignment and temperature, thus affecting laser performance.
In U.S. Pat. No. 5,226,051 to Chan et al. disclose how to equalize pulses in a Q-switched laser at different repetition rates in an open-loop system. In this arrangement information from a pulse is not used to predict or control the parameters of a subsequent pulse. Instead, the pump power to a given pulse is varied so that after a predetermined time a fixed amount of energy is stored in the laser medium. This method does reduce pulse variation with changes in repetition rate, but does not reduce the pulse-to-pulse variability at fixed repetition rates.
In view of the state of the art, there is a need for a pulse-to-pulse energy equalization system and method tailored specifically to pulsed lasers, such as Q-switched devices. The envisioned system should react quickly and take into account the non-linearities encountered in the repetition rate domain on the order of or greater than the inverse of the effective storage time (1/.tau.). Furthermore, the performance of the laser should not be impaired as a result.